1. Field of the Invention
Embodiments of the invention relate generally to the field of scanning electron microscopy (SEM). More particularly, embodiments of the invention relate to SEM techniques for imaging and measuring electronic transport in nanocomposites based on electric field induced contrast.
2. Discussion of the Related Art
Many new materials and devices envisioned in the near future will be based on using relatively long and slender conductive structures with unique electronic properties. These structures, commonly referred to as nanotubes, nanofibers, or nanowires can be used as additives to bulk materials to enhance the properties of the bulk material or add valuable properties to the bulk material. Nanowires may also be used individually to form the basic building blocks for next generation transistors or electron emitters. In any of these cases, progress requires knowledge of electronic transport properties within and between these nanowires as well as their influence on the electronic properties of the bulk material in which they reside.
Determining the location and accessing electronic transport information through nanomaterial percolation networks is difficult due to their small size and their fragile nature. This problem is exacerbated in the case of nano-composites since direct access is inhibited by a surrounding matrix. Few tools currently exist that have the ability to reveal, in-situ, the distribution of electric potential throughout these regions on such a small scale. Recent advances in scanning probe techniques, such as scanning impedance microscopy (SIM), offer possibilities, but with some limitations. One problem inherent with SIM, as with nearly all other scanning probe techniques, is the probe itself. The shape and quality of the tip can profoundly influence measurements. Reliability becomes a problem since shape and quality can vary from tip to tip and can change during even a single scan due to frictional wear or contamination. In addition, a high quality scan can take several minutes to capture, and in this time, the sample can drift causing distortion in the final image. The process of scanning probe microscopy becomes somewhat of an art since the operator has to find a balance between scanning slowly in order to reduce noise and increase resolution, and scanning quickly enough to avoid image distortion and tip degradation. Also, even though SIM has been shown to be capable of imaging nanowires inside a composite, the remarkable resolution commonly associated with scanning probe techniques is greatly diminished by the presence of a polymer.
Surface bias imaging using an SEM is know to those of skill in the art of scanning electron microscopy, and it is commonly referred to as ‘voltage contrast’. This term has come to encompass two main types of measurements. In using the terminology of Seiler, voltage contrast I is based on using sample biases on the order of 100's or 1000's volts to influence the trajectory of primary electrons. Such large voltages are not useful for measuring potential distributions in nanowires since the current flow associated with such high currents would destroy them. Voltage contrast II requires acquiring and measuring shifts in spectral energy distributions of secondary electrons and is not applicable for making maps of potential since the spectra are not taken locally. Voltage contrast II is instead useful only in measuring the average surface potential over the entire scan area in the SEM.
In summary, scanning probe measurements are slow, difficult to perform, plagued by reliability issues, and can not always provide sufficient resolution. Currently available SEM techniques can not offer adequate voltage and spatial resolution simultaneously.